Slurry casting or rheocasting is a procedure in which molten material is subjected to vigorous agitation as it undergoes solidification. During normal (i.e. non-rheocasting) solidification processes, dendritic structures form within the material that is solidifying. In geometric terms, a dendritic structure is a solidified particle shaped like an elongated stem having transverse branches. Vigorous agitation of materials, especially metals, during solidification eliminates at least some dendritic structures. Such agitation shears the tips of the solidifying dendritic structures, thereby reducing dendrite formation. The resulting material slurry is a solid-liquid composition, composed of solid, relatively fine, non-dendritic particles in a liquid matrix (hereinafter referred to as a semi-solid material).
At the molding stage, it is well known that components made from semi-solid material possess great advantages over conventional molten metal formation processes. These benefits derive, in large part, from the lowered thermal requirements for semi-solid material manipulation. A material in a semi-solid state is at a lower temperature than the same material in a liquid state. Additionally, the heat content of material in the semi-solid form is much lower. Thus, less energy is required, less heat needs to be removed, and casting equipment or molds used to form components from semi-solids have a longer life. Furthermore and perhaps most importantly, the casting equipment can process more material in a given amount of time because the cooling cycle is reduced. Other benefits from the use of semi-solid materials include more uniform cooling, a more homogeneous composition, and fewer voids and porosities in the resultant component.
The prior art contains many methods and apparatuses used in the formation of semi-solid materials. For example, there are two basic methods of effectuating vigorous agitation. One method is mechanical stirring. This method is exemplified by U.S. Pat. No. 3,951,651 to Mehrabian et al. which discloses rotating blades within a rotating crucible. The second method of agitation is accomplished with electromagnetic stirring. An example of this method is disclosed in U.S. Pat. No. 4,229,210 to Winter et al., which is incorporated herein by reference. Winter et al. disclose using either AC induction or pulsed DC magnetic fields to produce indirect stirring of the semi-solid.
Once the semi-solid material is formed, however, virtually all prior art methods then include a solidifying and reheating step. This so-called double processing entails solidifying the semi-solid material into a billet. One of many examples of double processing is disclosed in U.S. Pat. No. 4,771,818 to Kenney. The resulting solid billet from double processing is easily stored or transported for further processing. After solidification, the billet must be reheated for the material to regain the semi-solid properties and advantages discussed above. The reheated billet is then subjected to manipulation such as die casting or molding to form a component. In addition to modifying the material properties of the semi-solid, double processing requires additional cooling and reheating steps. For reasons of efficiency and material handling costs, it would be quite desirable to eliminate the solidifying and reheating step that double processing demands.
U.S. Pat. No. 3,902,544 to Flemings et al., incorporated herein by reference, discloses a semi-solid forming process integrated with a casting process. This process does not include a double processing, solidification step. There are, however, numerous difficulties with the disclosed process in Flemings et al. First and most significantly, Flemings et al. require multiple zones including a molten zone and an agitation zone which are integrally connected and require extremely precise temperature control. Additionally, in order to produce the semi-solid material, there is material flow through the integrally connected zones. Semi-solid material is produced through a combination of material flow and temperature gradient in the agitation zone. Thus, calibrating the required temperature gradient with the (possibly variably) flowing material is exceedingly difficult. Second, the Flemings et al. process discloses a single agitation means. Thorough and complete agitation is necessary to maximize the semi-solid characteristics described above. Third, the Flemings et al. process is lacking an effective transfer means and flow regulation from the agitation zone to a casting apparatus. Additional difficulties with the Flemings process, and improvements thereupon, will be apparent from the detailed description below.
A primary object of the present invention is to provide semi-solid material formation suitable for fashioning directly into a component.
Another object of the present invention is to provide a more efficient and cost-effective semi-solid material formation process.
Yet another object of the present invention is to provide an apparatus and a process for forming semi-solid material and maintaining the semi-solid material under substantially isothermal conditions.
An additional object of the present invention is to provide formation of semi-solid material suitable for component formation without a solidification and reheating step.
Still another object of the present invention is to provide a process and apparatus for semi-solid material formation with improved shearing and agitation.